1. Field of the Invention
The present invention relates to a light-emitting device including an electroluminescent material.
2. Related Art
A semiconductor laser has advantages of being extremely manufactured into a small-sized and lightweight laser oscillator compared with another gas laser or solid laser. A semiconductor laser has been put to practical use in various fields as an light source for transmitting and receiving signals by means of an optical interconnection in an optical integrated circuit; for recording on an optical disk, optical memory, or the like; for optical communication with an optical fiber as a light guide. The oscillation wavelength of a semiconductor laser has the wide range of blue to infrared. Most oscillation wavelengths of generally used semiconductor lasers are in an infrared region such as a GaAs laser (wavelength 0.84 μm), an InAs laser (wavelength 3.11 μm), an InSb laser (wavelength 5.2 μm), a GaAlAs (wavelength 0.72 to 0.9 μm), or an InGaAsP (wavelength 1.0 to 1.7 μm).
In recent years, research on the practical application of a semiconductor laser with an oscillation wavelength in a visible region has been intensified. A laser oscillator (organic semiconductor laser) that can produce laser oscillation including an electroluminescent material that can generate luminescence (electroluminescence) by being applied with an electric field has been attracted attention. Such an organic semiconductor laser can emit a visible light wavelength, and can be manufactured over a glass substrate at low cost. Accordingly, such an organic semiconductor laser is expected to be used for various purposes.
An organic semiconductor laser with a peak length λ at 510 nm is disclosed in unexamined patent publication No. 2000-156536 (p. 11).
In order to obtain stimulated emission from an organic semiconductor layer, strong energy that can make a population inversion (pumping energy) is required to be provided to an electroluminescent layer that serves as a laser medium. Specifically, pumping energy can be provided by applying a forward bias voltage to an anode and a cathode to supply a current to the electroluminescent layer interposed between the anode and the cathode. To produce laser oscillation practically, it is inadequate only to make a population inversion. It is required that a required pumping energy exceeds its threshold value to be provided to an electroluminescent layer that serves as a laser medium at the start of oscillation, so that stimulated emission exceeds all of internal losses such as absorption in a resonator.
Oscillation efficiency of laser light for the provided energy (electricity) becomes higher with decreasing pumping energy that is required at the start of oscillation, accordingly, power consumption can be reduced. Therefore, it has been expected an organic semiconductor laser that can improve oscillation efficiency of laser light with controlling pumping energy to be provided. Especially, the demand for high oscillation efficiency of laser light is increasing in the field where power consumption is directly linked to the commercial value.